The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk, and when the disk rotates, air adjacent to the disk moves along with the surface of the disk. The slider flies over the surface of the disk on a cushion of this moving air. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head has traditionally included a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic transitions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In present read head designs, a TMR sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a tunneling barrier layer, sandwiched between first and second ferromagnetic layers, referred to as a reference and a free layer. First and second leads are connected to the sensor for applying a sense voltage across the barrier. The magnetization of the reference layer is fixed perpendicular to the air bearing surface (ABS) and the magnetization of the free layer is oriented parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the reference layer is fixed by either direct exchange-pinning with an antiferromagnetic layer, or by strong antiferromagnetic coupling to a third ferromagnetic “pinned” layer which is exchange-pinned by an antiferromagnetic layer.
When the magnetizations of the reference and free layers are parallel with respect to one another, tunneling current across the barrier is maximized. When the magnetizations of the reference and free layer are antiparallel, tunneling current is minimized. The change in conductance of the TMR varies as cos θ, where θ is the angle between the magnetizations of the reference and free layers. In a read mode the resistance of the TMR sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense voltage is applied to the TMR sensor, resistance changes cause current changes that are detected and processed as playback signals.
More recently researchers have developed perpendicular magnetic recording systems. Older longitudinal recording system, such as one that incorporates the write head described above, stores data as magnetic bits oriented longitudinally along a track in the plane of the surface of the magnetic disk. This longitudinal data bit is recorded by a fringing field that forms between the pair of magnetic poles separated by a write gap.
A perpendicular recording system, by contrast, records data as magnetizations oriented perpendicular to the plane of the magnetic disk. The magnetic disk has a magnetically soft underlayer covered by a thin magnetically hard top layer. The perpendicular write head has a write pole with a very small cross section and a return pole having a much larger cross section. A strong, highly concentrated magnetic field emits from the write pole in a direction perpendicular to the magnetic disk surface, magnetizing the magnetically hard top layer. The resulting magnetic flux then travels through the soft underlayer, returning to the return pole where it is sufficiently spread out and weak that it will not erase the signal recorded by the write pole when it passes back through the magnetically hard top layer on its way back to the return pole. While the advent of perpendicular magnetic data recording systems have provided advances in increasing data density, still further increases in data density are needed.
As the areal density of recording increases, the size of the read sensor decreases. Read sensor technology such as TMR was introduced when the technology preceding it was not able to deliver the necessary signal and signal-to-noise ratio at the necessarily smaller sensor sizes. Similarly, TMR read sensors may find a limited range of device size (and hence limited areal density) below which it too may be inadequate to achieve necessary signal-to-noise performance.